In a semiconductor manufacturing process, there is used a plasma processing apparatus configured to supply a gas into a processing chamber and perform a certain process such as a film forming process or an etching process on a processing target substrate (hereinafter, simply referred to as a “substrate”) such as a semiconductor wafer, a liquid crystal substrate, or a substrate for a solar cell.
In a gas supply system of such a plasma processing apparatus, when supplying a mixture of multiple kinds of gases, multiple gas supply paths for supplying the multiple kinds of gases respectively are provided. Further, the multiple gas supply paths are joined to a single common gas supply path through which the processing gases are introduced into the processing chamber of the plasma processing apparatus.
A flow rate controller such as a mass flow controller (MFC) is provided on each of the gas supply paths, and opening/closing valves are provided at an upstream side and a downstream side of the flow rate controller, respectively. Further, the opening/closing valve may be provided only at the downstream side of the flow rate controller.
Conventionally, the supply of each processing gas is turned on and off by opening and closing each of the opening/closing valves, and a flow rate of the processing gas flowing in each of the gas supply paths is controlled by setting a certain flow rate of the flow rate controller (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Laid-open Publication No. 2002-85962    Patent Document 2: Japanese Patent Laid-open Publication No. H11-195641    Patent Document 3: Japanese Patent Laid-open Publication No. 2000-306887
Recently, as processes are diversified and there is an increasing demand for the miniaturization of semiconductor devices, various processes may be consecutively performed in a single apparatus while switching different kinds of processing gases. In such a case, if the supply of each processing gas is turned on and off by opening and closing the opening/closing valve of the mass flow controller, a flow rate control by the mass flow controller is started after a residual gas in the mass flow controller is once introduced into the processing chamber.
Accordingly, when switching the processing gases, a transient phenomenon may occur, causing a problem that a flow rate of the gas supplied into the processing chamber is not stabilized. The transient phenomenon of the gas flow rate tends to occur more readily as a time interval for switching the gases is shorter.
To solve the problem, by performing a process for stabilizing the gas flow rate each time the gases are switched, the transient phenomenon of the gas flow rate may be suppressed. However, in such a case, a throughput may be deteriorated.
Further, it is disclosed in Patent Documents 2 and 3 that a transition process is provided when switching an etching gas and a deposition gas alternatively. In this transition process, before stopping the supply of one gas, the supply of the other gas is started, and a flow rate of the one gas is gradually decreased while a flow rate of the other gas is gradually increased. According to the flow rate control of Patent Documents 2 and 3, however, the alternately supplied gases are mixed when the gases are switched. In such a case, depending on the kind of the switched gases, an adverse effect on the substrate process may not be negligible.